"perplexed disbelief on the inspectors"
Now you're tempting me, KJ ...
I believe the two screw requirement does apply. I've seen those covers in 4/0 size, which will mount on a round plastic box. It won'nt be pretty, but it does the job.
For an earlier poster, I don't think the NEC uses the term 'lampholder' any more, except for something that is directly mounted on the end of a cord (temporary lighting festoons). They're all 'luminaires,' even the keyless ones.
